Thermosetting epoxy resins have been widely applied in several fields such as building, coating material, electronic equipment, and cellulose composite material. However, the thermosetting epoxy resin will be a polymer network of high crosslink density, and it has problems such as brittleness and poor impact resistance. With recent developments of composite material, coating material, and electronic equipment, a thermosetting epoxy resin with a better thermal stability is especially demanded. The thermal stability of the thermosetting epoxy resin is generally enhanced by increasing its crosslink density, but it may make the epoxy resin be more brittle with a lower impact resistance. Therefore, enhancing the toughness of the epoxy resin is an important topic for development. The toughness of the epoxy resin can be enhanced by adding carboxyl terminated butadiene acrylonitrile (CTBN), but it may lower the thermal stability (glass transfer temperature) of the epoxy resin. Accordingly, a method of simultaneously improving the toughness and keeping the thermal stability of the epoxy resin is called-for.